1. Field of the Invention
The present invention relates to a microwave oscillator circuit, and more specifically to a microwave oscillator circuit such as a local oscillator for use in a frequency converter in a radio communication device.
2. Description of the Prior Art
Use is made conventionally, as a first local oscillator for use in a microwave communication device, of an FET oscillator equipped with a resonator circuit, oscillation frequency of which FET oscillator is defined by a resonance frequency of the resonator. Such microwave communication devices require varieties of local oscillation frequencies. For obtaining a desired oscillation frequency signal there are known two techniques: (1) one in which the local oscillator is directly brought into oscillation at a required frequency, and (2) the other in which it is previously brought into oscillation at a frequency 1/n times the required frequency and an oscillated signal is multiplied by n through a multiplying unit.
In the case where a k band or higher frequency signal whose direct oscillation is difficult is required, use is generally made of the foregoing latter technique (2). The latter technique is sometime employed also for cases where frequency stabilization is achieved by the use of a PLL, although a care should be taken for the even balance of the costs of a frequency divider and a multiplier and characteristics thereof. The multiplier basically doubles associated frequency and generally multiplies the frequency by 2 m (m: number of stages) in a multi-stage construction thereof.
Referring to FIG. 1, a prior art circuit equipped with a local oscillator and a multiplier is exempiarily illustrated. A section A indicates the local oscillator and a section B indicates the multiplier. The local oscillator A includes an FETQ1 as an FET for oscillation whose oscillatable frequency range is determined by a reactance element L1 and a capacitance element C3 connected to the drain side of FETQ1 and by a capacitance element C6 connected to the source side of FETQ1. A resonator circuit 1 is connected to a gate of the FETQ1, whereby the FETQ1 is brought into oscillation at a resonance frequency of the resonator 1. Elements L2 and L5 serve respectively as a bias feed element and a DC return element of the FETQ1 and provide choke circuits each having sufficiently high impedances to the oscillation frequency. Further, elements R2 and R11 are set to determine proper operating points as a self-bias circuit of the FETQ1.
An oscillation signal from the FETQ1 is inputted to the multiplier B through a capacitor C7 which serves to determine the extent of cutting of a DC component and the degree of coupling. The multiplier B comprises a first filter 4, a second filter 5, and a diode D1. The diode 1 produces spurious components of the input signal owing to non-linearity thereof, and a second harmonic component of the input signal can effectively be derived from an output terminal 8 by the use of the first filter 4 as a filter through which an input signal frequency can pass and further by the use of the second filter 5 as a filter through which a frequency twice that of the input signal can pass.
A bias circuit for the diode D1 comprises a choke circuit L6 and a resistor R12, which may be employed for applying bias voltage to the diode D1 and for simply providing a DC return not for bias feeding. In both cases the bias circuit operates as choke circuits.
The prior art microwave oscillator circuit constructed as above, however, suffers from difficulties: It requires bias circuits for the local oscillator A and the multiplier B because they are independently provided. In particular, for the local oscillator it requires L5 and L11 for the DC return. It is thereupon needed to realize high impedance with L5 because R11 is a source resistance which can not be a high resistance. This causes the microwave oscillator circuit to be large-sized. Particularly, construction of the microwave oscillator circuit as an MMIC (monolithic microwave IC) causes, if desired, the area of a passive element to be increased, which is disadvantageous in cost in comparison with discrete constructions and makes it difficult to construct the circuit as the MMIC.